Achievements
Complete each achievement to get the allotted gamerscore:

101 Cremations (20) - Set 101 Infected on fire.
Akimbo Assassin (30) - Survive an entire campaign using only pistols.
All 4 Dead (30) - Kill all four Survivors in one life while playing as a Tank.
Back 2 Help (20) - Leave a safe room to save an incapped teammate and bring them back safely.
Barf Bagged (20) - Cover four Survivors with Boomer bile at once.
Big Drag (20) - Drag a Survivor 100 feet with your tongue.
Blind Luck (20) - You or another Survivor take no damage after being vomited on by a Boomer.
Brain Salad (15) - Make 100 headshot kills.
Burn the Witch (10) - Light a Witch with a Molotov.
Chain Smoker (20) - Constrict two Survivors in one life as a Smoker.
Clean Kill (10) - Shove a Boomer and then kill him without him splashing on anyone.
Cr0wnd (30) - Kill a Witch with a single headshot.
Dead Giveaway (10) - Heal a fellow Survivor when your own health is below 10.
Dead Stop (10) - Punch a Hunter as he is pouncing.
Dead Wreckening (20) - Dole out 5000 total Survivor damage as a Special Infected.
Dead Baron (15) - Survive the Dead Air campaign.
Do Not Disturb (20) - Sneak past all Witches in a campaign without disturbing one.
Double Jump (20) - Pounce two different Survivors in one life as a Hunter.
Drag and Drop (20) - Rescue a Survivor from a Smoker's tongue before he takes damage.
Field Medic (20) - Heal 25 Survivors with a first aid kit.
Grim Reaper (15) - Survive the Blood Harvest campaign.
Jump Shot (20) - Headshot a Hunter while he's leaping.
Lamb 2 Slaughter (20) - As an Infected, incap a Survivor who has entered and left a safe room.
Ground Cover (30) - Save another Survivor from a Special Infected while on the ground.
Helping Hand (20) - Revive 50 incapacitated Survivors.
Hero Closet (10) - Rescue a Survivor trapped in a closet.
Hunter Punter (10) - Shove a Hunter off of a pinned and helpless Survivor.
Tankbusters (20) - Kill a Tank without it dealing any damage to a Survivor.
Toll Collector (15) - Survive the Death Toll campaign.
Tongue Twister (20) - Free yourself from a Smoker who has grabbed you with his tongue.
Man vs Tank (30) - Single-handedly kill a Tank.
Mercy Killer (15) - Survive the No Mercy campaign.
My Bodyguard (15) - Protect any Survivor from an attacking Infected 50 times.
No Smoking Section (15) - Kill 10 Smokers as they are pulling helpless Survivors.
No-one Left Behind (20) - Beat a campaign with all 4 Survivors.
Nothing Special (30) - Survive a campaign with no Survivors taking damage from Special Infected.
Pharm-assist (20) - Give pain pills to 10 Survivors.
Pyrotechnician (20) - Blow up 20 Infected in a single explosion.
Red Mist (20) - Kill 1000 Infected with a mounted machine gun.
Safety First (30) - Play an entire campaign with no Survivors taking friendly fire damage.
Spinal Tap (10) - Kill an Infected with a single blow from behind.
Stand Tall (20) - Survive a campaign without being incapacitated.
Stomach Upset (20) - All Survivors complete a campaign without being vomited on.
Towering Inferno (10) - Light a Tank with a Molotov.
Unbreakable (30) - Finish a campaign without ever being healed.
Untouchables (30) - No Survivors take damage after contacting the rescue vehicle.
What Are You Trying to Prove? (35) - Survive all campaigns on Expert.
Witch Hunter (20) - Kill a Witch without any Survivor taking damage from her.
Zombicidal Maniac (30) - Survive any campaign on Expert.
Zombie Genocidest (20) - Kill 53,595 Infected.

Additionally, there are ten Crash Course DLC achievements:

Jumpin' Jack Smash (25) - Pounce a Survivor for 25 points of damage in the Crash Course campaign.
Quick Power (25) - Restart the generator within 30 seconds of it shutting off in the Crash Course campaign.
Slippery Pull (25) - Smoker pull a bile-covered Survivor until you hold him during Crash Course.
20 Car Pile-up (20) - As a Tank hit 20 Survivors with a car in the Crash Course campaign.
Crash-proof (15) - Survive the Crash Course Campaign.
Smash Hit (25) - Win a Versus campaign of Crash Course.
Tank Stumble (20) - Stun a Tank with an explosion in the Crash Course campaign.
The Littlest Genocide (25) - Kill 5,359 Infected in the Crash Course campaign.
Truck Stop (35) - Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
Wipefest (35) - Your team incapacitates three Survivors within five seconds in the Crash Course campaign.

Additionally, there five The Sacrifice DLC achievements:

Kill Bill (50) - Have Bill sacrifice himself for the team.
Barrel Rolled (50) - Kill a Special Infected with an exploding barrel.
Chaos Generator (50) - Have all 3 generators running at once in "The Sacrifice" finale.
Sacrifizzle (50) - As a Special Infected, incap someone who is trying to sacrifice themselves.
Supreme Sacrifice (50) - Complete "The Sacrifice".

Akimbo Assassin Achievement Made Easy
The Akimbo Assassin Achievement requires you to play an entire mission with only the pistol(s). One easy way to complete this is to start a mission, shoot a few zombies with your pistol only, then choose "Take a Break" and let the NPCs finish the level.

Speed Life
As the director's commentary points out, the Tank can be outrun by any survivor with green (51% or more) health, but the Tank can overtake any survivor with 50% of less life, because the more injured a survivor is, the slower s/he moves.

Pain pills will boost the speed of an injured survivor, although the color of the life bar will reflect his/her original health status.

Cr0wnd Achievement
To kill a witch with a headshot, put the game on Easy difficulty, acquire an auto-shotgun, and look for a witch. The player going after this achievement should simply (and quickly) approach the witch (lights off), causing her to rise. As the witch is standing (she is slower to attack on easy), unload all 10 shells into her head.

Hot Swap
If you need to 'reset' your primary weapon to its original (cherry) state, pick up a weapon different from yours, then re-select the weapon you want. This is faster than reloading (plus the time used from replenishing from the ammunition pile), and can help in close-call rescue finales.

Respawn for Max Health in Vs.
If you'd like to "refill" your health as a zombie, run away from the survivors until you see a message claiming you are too far away. Press X, and then you'll be allowed to respawn back in the fray. Hopefully they won't make it to the safe room before you're back!

Unbreakable Achievement
The Unbreakable Achievement requires you to not heal yourself or be healed throughout an entire mission. If the computer begins to heal you, whip out your own health pack (don't heal yourself though!) and it will stop the computer in its tracks.